The present invention relates to a hydrocarbon conversion process. More particularly, this invention relates to the catalytic hydrocracking of hydrocarbons.
The hydrocracking of hydrocarbons is old and wellknown in the prior art. These hydrocracking processes can be used to hydrocrack various hydrocarbon fractions such as reduced crudes, gas oils, heavy gas oils, topped crudes, shale oil, coal extract and tar extract wherein these fractions may or may not contain nitrogen compounds. Modern hydrocracking processes were developed primarily to process feeds having a high content of polycyclic aromatic compounds, which are relatively unreactive in catalytic cracking. The hydrocracking process is used to produce desirable products such as turbine fuel, diesel fuel, and middle distillate products such as naphtha and gasoline.
The hydrocracking process is generally carried out in any suitable reaction vessel under elevated temperatures and pressures in the presence of hydrogen and a hydrocracking catalyst so as to yield a product containing the desired distribution of hydrocarbon products
Hydrocracking catalysts generally comprise a hydrogenation component on an acidic cracking support. More specifically, hydrocracking catalysts comprise a hydrogenation componett selected from the group consisting of Group VIB metals and or Group VIII metals of the Periodic Table of Elements, their oxides or sulfides. The prior art has also taught that these hydrocracking catalysts contain an acidic support comprising a crystalline aluminosilicate material such as X-type and Y-type aluminosilicate materials. This crystalline aluminosilicate material is generally suspended in a refractory inorganic oxide such as silica, alumina, or silica-alumina.
Regarding the hydrogenation component the preferred Group VIB metals are tungsten and molybdenum; the preferred Group VIII metals are nickel and cobalt. The prior art has also taught that combinations of metals for the hydrogenation component, expressed as oxides and in the order of preference, are: NiO-WO.sub.3, NiO-MoO.sub.3, CoO-MoO.sub.3, and CoO-WO.sub.3. Other hydrogenation components broadly taught by the prior art include iron, ruthenium, rhodium, palladium, osmium, indium, platinum, chromium, vanadium, niobium, and tantalum.
References that disclose hydrocracking catalysts utilizing nickel and tungsten as hydrogenation components, teach enhanced hydrocracking activity when the matrix or catalyst support contains silica-alumina. For instance, U.S. Pat. Nos. 4,576,711, 4,563,434, and 4,517,073 all to Ward et al., show at Table V thereof that the lowest hydrocracking activity is achieved when alumina is used in the support instead of a dispersion of silica-alumina in alumina. The lowest hydrocracking activity is indicated by the highest reactor temperature required to achieve 60 vol. % conversion of the hydrocarbon components boiling above a predetermining end point to below that end point.
Similarly, U.S. Pat. No. 3,536,605 to Kittrell et al. teaches the use of silica-alumina in the catalyst support when a nickel- and tungsten-containing hydrogenation component is employed.
U.S. Pat. No. 3,598,719 to White teaches a hydrocracking catalyst that can contain 0 wt. % silica, i.e. less than 15 wt. % silica. All of the examples, however, show the presence of silica and there is no disclosure of a catalyst containing an alumina matrix when the hydrogenation metals are specifically nickel and tungsten.
As can be appreciated from the above, there is a myriad of catalysts or catalyst systems known for hydrocracking whose properties vary widely. A catalyst suitable for maximizing naphtha yield may not be suitable for maximizing the yield of turbine fuel or distillate. Further, the various reactions; i.e., denitrogenation, hydrogenation, and hydrocracking must be reconciled in a hydrocracking process in an optimum manner to achieve the desired results.
For instance when a feedstock having a high nitrogen content is exposed to a hydrocracking catalyst containing a high amount of cracking component the nitrogen serves to poison or deactivate the cracking component. Thus, hydrodenitrogenation catalysts do not possess a high cracking activity since they are generally devoid of a cracking component that is capable of being poisoned. Another difficulty is presented when the hydrocracking process is used to maximize naphtha yields from a feedstock containing light catalytic cycle oil which has a very high aromatics content. The saturation properties of the catalyst must be carefully gauged to saturate only one aromatic ring of a polynuclear aromatic compound such as naphthalene in order to preserve desirable high octane value aromatic-containing hydrocarbons for the naphtha fraction. If the saturation activity is too high, all of the aromatic rings will be saturated and subsequently cracked to lower octane value paraffins.
On the other hand, distillate fuels such as diesel fuel or aviation fuel have specifications that stipulate a low aromatics content. This is due to the undesirable smoke production caused by the combustion of aromatics in diesel engines and jet engines.
Prior art processes designed to convert high nitrogen content feedstocks are usually two stage processes wherein the first stage is designed to convert organic nitrogen compounds to ammonia prior to contacting with a hydrocracking catalyst which contained a high amount of cracking component; e.g., a molecular sieve material.
For instance U.S. Pat. No. 3,923,638 to Bertolacini et al., discloses a two catalyst process suitable for converting a hydrocarbon containing substantial amounts of nitrogen to saturated products adequate for use as jet fuel. Specifically, the subject patent discloses a process wherein the hydrodenitrogenation catalyst comprises as a hydrogenation component a Group VIB metal and Group VIII metal and/or their compounds and a cocatalytic acidic support comprising a large-pore crystalline aluminosilicate material and refractory inorganic oxide. The hydrocracking catalyst comprises as a hydrogenation component a Group VIB metal and a Group VIII metal and/or their compounds, and an acidic support of large-pore crystalline aluminosilicate material. For both hydrodenitrogenation catalyst and the hydrocracking catalyst, the preferred hydrogenation component comprises nickel and tungsten and/or their compounds and the preferred large-pore crystalline aluminosilicate material is ultrastable, large-pore crystalline aluminosilicate material.
In accordance with the present invention it has been discovered that the naphtha yield of a hydrocracking process can be markedly increased by employing a plurality of reaction zones in series wherein each zone contains a particular catalyst and when these catalysts are juxtaposed in the zones in an essential order. Specifically, it has been discovered that even if the same volumes or weights of particular catalysts are used in the zones of a hydrocracking process, if the zones are not juxtaposed in accordance with the present invention the increased naphtha yield will not be afforded.
An attendant advantage of the process of the present invention is an increase in overall catalyst activity.